Fundamental characters of phylum chordates:
Presence of notochord: They are characterized by the presence of rod-like supporting notochord extending throughout the length of the body.
Presence of dorsal tubular nerve cord: They are characterized by the presence of dorsal nerve cord lying above the notochord.
Presence of paired pharyngeal gill slits: They are characterized by the presence of paired pharyngeal gill slits at some stage of their lifetime.
Presences of ventral heart: They are characterized by a ventral heart, i.e. heart is situated on the ventral side of the alimentary canal in the body cavity.
Presence of post-anal tail.
Body cavity: They are coelomates, Triploblsatic body, and bilaterally symmetrical animals.
Phylum Chordata is divided into three sub-phyla:
Urochordata (Gr: aura-tail): Here, the notochord is confined only to the tall region of the larva. It is represented by primitive chordates like Ascidia, Herdmania, commonly called ‘sea squirt’.
Cephalochordate (Gr: cephalon-head): Here, the notochord is well-developed, extending from the anterior to the posterior region of the body. Anteriorly, it extends beyond the nerve cord. Hence, the name cephalochordate. It includes primitive chordates like Amphioxus. Since the above two subphyla include primitive chordates, they are also called Protochordata.
Vertebrata or Craniata: Here, the notochord is replaced by the vertebral column.